Ganglion cells in the retina may project to either the ipsilateral or contralateral side of the brain, depending upon their class and on their retinal position. The long-long-term objective is to understand the determinants of the "decussation pattern" of the distinct retinal ganglion cell classes in mammals. The specific aim of this proposal is to define the relative roles of two hypothesized mechanisms: 1) decussation patterns are produced as a consequence of regressive events that selectively eliminate cells projecting to the inappropriate side of the brain; or 2) decussation patterns are produced as a consequence of selective axonal navigation at the chiasmatic region, where some timeand/or position-dependent event determines the side of choice. Studies will use the visual systems of adult and developing cats and ferrets, in which there are well-characterized differences in decussation patterns between the retinal ganglion cell classes and between the species. These species and cell class differences are the basis for the proposed experiments. The experimental approach employed is to assess either the nature of the crossed and uncrossed ganglion cell distributions on the retina (the "decussation pattern"), or to determine the organization of optic fibers in the chiasmatic region, either a) during development, b) in the normal adult, or c) as a consequence of some early manipulation to the visual pathway. The experiments will make use of the techniques of Horseradish peroxidase histochemistry or carbocyanine dye diffusion, with which cells in the temporal retina can be identified according to their side of projection; tritiated thymidine autoradiography, with which the order of genesis of the retinal cells projecting to the different hemispheres can be determined; myelin staining of axonal classes, with which the fiber organization of the adult chiasm can be determined; and electron microscopy, with which the position of growth cones at different regions in the developing chiasmatic region can be defined. Finally, a novel tissue culture technique is described for studying chiasmatic pathway decisions during development.